Selective Degradation of Polo-like Kinase 1 by a Hydrophobically Tagged Inhibitor of the Polo-Box Domain.

Hydrophobic tagging (HT) of bioactive compounds can induce target degradation via the proteasomal pathway. The first application of hydrophobic tagging to an existing inhibitor of protein-protein interactions is now presented. We developed Poloxin-2HT by fusing an adamantyl tag to Poloxin-2, an inhibitor of the polo-box domain of the protein kinase Plk1, which is a target for tumor therapy. Poloxin-2HT selectively reduced the protein levels of Plk1 in HeLa cells and had a significantly stronger effect on cell viability and the induction of apoptosis than the untagged PBD inhibitor Poloxin-2. The change in cellular phenotype associated with the addition of the hydrophobic tag to Poloxin-2 demonstrated that Poloxin-2HT targets Plk1 in living cells. Our data validate hydrophobic tagging of selective inhibitors of protein-protein interactions as a novel strategy to target and destroy disease-relevant proteins.

Development of Bifunctional Inhibitors of Polo-Like Kinase†1 with Low-Nanomolar Activities Against the Polo-Box Domain.

Polo-like kinase 1 (Plk1), a validated cancer target, harbors a protein-protein interaction domain referred to as the polo-box domain (PBD), in addition to its enzymatic domain. Although functional inhibition either of the enzymatic domain or of the PBD has been shown to inhibit Plk1, so far there have been no reports of bifunctional agents with the potential to target both protein domains. Here we report the development of Plk1 inhibitors that incorporate both an ATP-competitive ligand of the enzymatic domain, derived from BI 2536, and a functional inhibitor of the PBD, based either on the small molecule poloxin-2 or on a PBD-binding peptide. Although these bifunctional agents do not seem to bind both protein domains simultaneously, the most potent compound displays low-nanomolar activity against the Plk1 PBD, with excellent selectivity over the PBDs of Plk2 and Plk3. Our data provide insights into challenges and opportunities relating to the optimization of Plk1 PBD ligands as potent Plk1 inhibitors.

Optimized Plk1 PBD Inhibitors Based on Poloxin Induce Mitotic Arrest and Apoptosis in Tumor Cells.

Polo-like kinase 1 (Plk1) is a central regulator of mitosis and has been validated as a target for antitumor therapy. The polo-box domain (PBD) of Plk1 regulates its kinase activity and mediates the subcellular localization of Plk1 and its interactions with a subset of its substrates. Functional inhibition of the Plk1 PBD by low-molecular weight inhibitors has been shown to represent a viable strategy by which to inhibit the enzyme, while avoiding selectivity issues caused by the conserved nature of the ATP binding site. Here, we report structure-activity relationships and mechanistic analysis for the first reported Plk1 PBD inhibitor, Poloxin. We present the identification of the optimized analog Poloxin-2, displaying significantly improved potency and selectivity over Poloxin. Poloxin-2 induces mitotic arrest and apoptosis in cultured human tumor cells at low micromolar concentrations, highlighting it as a valuable tool compound for exploring the function of the Plk1 PBD in living cells.

Nanomolar inhibitors of the transcription factor STAT5b with high selectivity over STAT5a.

Src homology 2 (SH2) domains play a central role in signal transduction. Although many SH2 domains have been validated as drug targets, their structural similarity makes development of specific inhibitors difficult. The cancer-relevant transcription factors STAT5a and STAT5b are particularly challenging small-molecule targets because their SH2 domains are 93% identical on the amino acid level. Here we present the natural product-inspired development of the low-nanomolar inhibitor Stafib-1, as the first small molecule which inhibits the STAT5b SH2 domain (K(i)=44 nM) with more than 50-fold selectivity over STAT5a. The binding site of the core moiety of Stafib-1 was validated by functional analysis of point mutants. A prodrug of Stafib-1 was shown to inhibit STAT5b with high selectivity over STAT5a in tumor cells. Stafib-1 provides the first demonstration that naturally occurring SH2 domains with more than 90% sequence identity can be selectively targeted with small organic molecules.

Peptoid-Peptide hybrid ligands targeting the polo box domain of polo-like kinase 1.

We replaced the amino terminal Pro residue of the Plk1 polo-box-domain-binding pentapeptide (PLHSpT) with a library of N-alkyl-Gly "peptoids", and identified long-chain tethered phenyl moieties giving greater than two-orders-of-magnitude affinity enhancement. Further simplification by replacing the peptoid residue with appropriate amides gave low-nanomolar affinity N-acylated tetrapeptides. Binding of the N-terminal long-chain phenyl extension was demonstrated by X-ray co-crystal data.

Identification of high affinity polo-like kinase 1 (Plk1) polo-box domain binding peptides using oxime-based diversification.

In an effort to develop improved binding antagonists of the polo-like kinase 1 (Plk1) polo-box domain (PBD), we optimized interactions of the known high affinity 5-mer peptide PLHSpT using oxime-based post solid-phase peptide diversification of the N-terminal Pro residue. This allowed us to achieve up to two orders of magnitude potency enhancement. An X-ray crystal structure of the highest affinity analogue in complex with Plk1 PBD revealed new binding interactions in a hydrophobic channel that had been occluded in X-ray structures of the unliganded protein. This study represents an important example where amino acid modification by post solid-phase oxime ligation can facilitate the development of protein-protein interaction inhibitors by identifying new binding pockets that would not otherwise be accessible to coded amino acid residues.